1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, and more particularly, to a tandem-type image forming apparatus that forms toner images on a plurality of latent image bearing members, transfers the toner images onto an intermediate transfer member on one another, and transfers the superimposed toner image onto a recording medium.
2. Description of the Background Art
Conventionally, there is known a tandem-type image forming apparatus equipped with an endlessly rotatable intermediate transfer belt that is wound around a plurality of rollers, process cartridges for colors yellow, magenta, cyan, and black arranged in a direction of movement of the intermediate transfer belt, and so forth.
Each of the process cartridges includes, for example, a photoreceptor serving as a latent image bearing member and a developing device. In such a process cartridge, the surface of the photoreceptor is illuminated with light so that an electrostatic latent image is formed on its surface. Then, the developing device develops the electrostatic latent image with a respective color of toner, thereby forming a visible image, known as a toner image.
The toner images of yellow, magenta, cyan, and black formed on the respective photoreceptor are superimposed on the intermediate transfer belt. Accordingly, a multi-color toner image is formed on the intermediate transfer belt. Lastly, the multi-color toner image is secondarily transferred onto a recording medium.
In such a tandem-type image forming apparatus, a problem, known as reverse transfer, may occur in all the process cartridges except for the process cartridge disposed substantially at the initial part, or upstream end, of the transfer process. For example, if the toner images of yellow, magenta, cyan, and black are sequentially superimposed, in that order, on the intermediate transfer belt, the process cartridge for yellow corresponds to the process cartridge disposed at the upstream end of the transfer process.
Except for the process cartridge for yellow, in the rest of the process cartridges, that is, the process cartridges for magenta, cyan, and black, the toner images transferred on the intermediate transfer belt upstream therefrom may be undesirably transferred back onto the photoreceptors again. This phenomenon is the so-called reverse transfer phenomenon (hereinafter simply “reverse transfer”). Such reverse transfer results in an abnormal multi-color image.
Various print experiments performed by the present inventor using a test machine similar to the known tandem-type image forming apparatuses showed that reverse transfer tended to occur when images were continuously formed on a relatively large number of recording media sheets when the image forming apparatus was operating in a continuous output mode.
Specifically, it was found that an electric potential of the intermediate transfer belt of the test machine increased gradually during the continuous output mode in which 500 sheets were output over a relatively long period of time, such that, when the potential of the intermediate transfer belt reached a certain level, electric discharge occurred in a space between the intermediate transfer belt and a device such as a process cartridge casing disposed opposite the belt, thereby degrading adhesion between the toner and the intermediate transfer belt.
As a result, the toner was reversely transferred from the intermediate transfer belt to the photoreceptors in the process cartridges disposed further downstream in the transfer process. Thereafter, the potential of the intermediate transfer belt continued rising until application of the transfer bias to the intermediate transfer belt stopped.
The present inventor also discovered that if application of the transfer bias relative to the intermediate transfer belt was temporarily halted, for example, for one minute, the belt potential returned to a normal level even after rising a certain amount due to the continuous output.